Within the semiconductor industry, there are numerous applications that require bonding a semiconductor wafer to a second wafer or glass, an example being sensors formed by a silicon wafer (referred to herein as a device wafer) with a micromachined sensing structure (micromachine), which is capped by a semiconductor or glass wafer (referred to herein as a capping wafer). Examples of semiconductor sensors include yaw (angular rate) sensors, accelerometers and pressure sensors, each of which typically entails a cavity that encloses the micromachine between the wafers. Absolute pressure sensors require that the cavity be evacuated and hermetically sealed, while the performance of yaw sensors with resonating micromachines generally benefit if the cavity is evacuated so that the micromachine operates in a vacuum.
By the very nature of their operation, micromachines must be free to move to some degree, necessitating that the seal between the wafers is sufficient to exclude foreign matter from the cavity. A hermetical seal ensures that moisture is also excluded, which would form ice crystals at low temperatures that could impede motion of the micromachine. Accordingly, the integrity of the bond between the wafers is essential to the life of a semiconductor sensor. Various bonding techniques have been used for the purpose of maximizing the strength and reliability of the bond. For example, the use of adhesives, dielectrics such as glass frit, and solders as intermediate bonding materials has been suggested in the prior art. Silicon direct and anodic bonding techniques that do not require an intermediate material have also been used. As would be expected, the conditions vary under which each of these bonding techniques will reliably yield a hermetic seal.
While each of these sealing methods has found wide use, sensors are inevitably produced whose cavities are not hermetically sealed after the bonding operation. To reduce returns and field failures, devices with inadequate seals need to be identified following the bonding operation. Labor intensive visual inspections can be useful to screen out unsealed devices, but are expensive and compromised by the likelihood of human error. Various automated inspection techniques are also available, though each have limitations and are often expensive to implement in a large-scale assembly process.